Truss fabrication systems typically include a truss assembly table and a gantry press. The truss assembly table includes several groups of planks forming the top of the table, and accessways or walkways spaced intermittently between the groups of planks providing operators access to portions of the table spaced from its edges. Operators place truss members and connector plates on the truss assembly table in predetermined configurations to form particular trusses. After the truss members and connector plates are placed on the table, the gantry press travels along the truss assembly table to press the connector plates into the truss members, joining them together. The gantry press typically includes a roller or hydraulic platen mounted on a gantry that engages the connector plates and presses them into the truss members. The gantry has wheels that run on rails or guides extending along the sides of the table or on the floor next to the table for guiding the gantry along the table.
Obstruction detection devices are mounted on the gantry press to detect whether obstructions are present on the table that may damage the press or be damaged by the press. For example, conventional obstruction detection devices for truss fabrication systems include push rods, light curtains, or light beams mounted on the gantry press above the table planks. If a push rod is contacted by an obstruction on the table, the push rods mechanically or electrically trigger a shut-off switch to stop the gantry press, preventing the obstruction from being contacted by the press. Similarly, if a light curtain or light beam is interrupted by an obstruction on the table, a shut-off switch is triggered to stop the gantry press and prevent the obstruction from being contacted by the press. However, if an obstruction is located beneath a level of the table planks, such as an operator bent over in a walkway extending between the planks of the table, the conventional push rods, light curtains, and light beams will not detect the obstruction. If an obstruction in a walkway rises to a level above the table planks after the push rods, light curtains, or light beams pass but before the roller passes, the roller may pinch the obstruction between the roller and truss members and/or connector plates positioned on the table planks, damaging the roller or the obstruction. Thus, there is a need for an obstruction detection device capable of detecting obstructions positioned in the walkways between the planks of the table.